Mobile communication systems that use orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) often use frequency allocation so as to use different frequency resources among adjacent cells because the use of the same frequency resource by adjacent cells at the same time causes mutual interference. In this case, dividing the frequency resources and allocating the divided different frequency resources to the individual cells results in an extremely small frequency resource that may be used by each cell. Therefore, the frequency resources are reused by repeatedly using the same frequency resource among cells that are not adjacent to one another.
For the frequency allocation, fractional frequency reuse (FFR) attracts attention as a technology for further increasing the frequency use efficiency. With FFR, a base station uses a frequency domain different from adjacent base stations (R3 zone: a frequency domain that uses one third of all subchannels to achieve a frequency reuse coefficient of 3) and a frequency domain common to adjacent base stations (R1 zone: a frequency domain that uses all subchannels to achieve frequency reuse coefficient of 1). The different frequency domain is used for communication with a mobile station with low radio quality located at the boundary of sectors or at the edge of a cell. The common frequency domain is used for communication with a mobile station with high radio quality located in the vicinity of the base station. This allows the frequency use efficiency to be increased.
If a mobile station located at the boundary of sectors uses the same frequency as that of adjacent sectors, the value of the preamble signal to interference and noise ratio (SINR) of the mobile station is small because of significant interference from the adjacent sectors. The preamble signal is a synchronizing signal sent by the sectors, for which the same frequency is used by all the sectors. Therefore, in selecting a zone for a mobile station based on the preamble SINR for FFR, the R3 zone is selected for a mobile station located at the boundary of the sectors.